Commitments and Contingencies - Rent expense and inventory purchase obligations (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Rent expense and inventory purchase commitments
Aggregate rent expense under operating leases	$ 3,806	$ 3,384	$ 2,998
Inventory
Rent expense and inventory purchase commitments
Inventory purchase obligation	$ 14,985

Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41499-112717

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- Definition

Amount of the recorded unconditional purchase obligation maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 440
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=77896938&loc=d3e25383-109308

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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